1. Field of the Invention
The present invention relates to a head substrate, printhead, head cartridge, and printing apparatus. Particularly, the present invention relates to a head substrate on which electrothermal transducers serving as heaters for generating heat energy necessary to print, and driving circuits for driving the electrothermal transducers are formed on a single substrate, a printhead using the head substrate, a head cartridge using the printhead, and a printing apparatus.
2. Description of the Related Art
Electrothermal transducers (heaters) and their driving circuits on a printhead mounted in a conventional inkjet printing apparatus are formed on a single substrate using a semiconductor process, as disclosed in, for example, Japanese Patent Laid-Open No. 5-185594. The “driving circuit” generically means a logic circuit, driver transistor, and the like for driving a heater. There has already been proposed a substrate of an arrangement in which an ink supply port for supplying ink is formed in the substrate and heaters are arrayed near the ink supply port to face each other.
FIG. 9 is a block diagram showing an example of the schematic layout of a head substrate used in a conventional inkjet printhead (to be referred to as a printhead hereinafter).
Referring to FIG. 9, a substrate 100 integrates heaters and their driving circuits by a semiconductor process. An ink supply port 101 supplies ink from the lower surface of the substrate. A heater array 102 includes a plurality of heaters. A driver transistor array 103 includes a plurality of driver transistors for supplying a desired current to heaters. A logic circuit 104a forms part of a driving circuit for generating a signal for selectively driving a driver transistor of the driver transistor array 103 for each desired heater block. A connection terminal 105 receives a power supply voltage and electrical signal from outside the substrate and outputs them to outside the substrate.
FIG. 10 is an equivalent circuit diagram of one segment for supplying a current to a heater in order to discharge ink.
Referring to FIG. 10, an AND circuit 701 calculates the logical product between a block selection signal sent from a decoder to select a block of heaters divided into desirable numbers of blocks, and a print data signal output from a shift register via a latch circuit. A level conversion circuit (LVC) 702 converts the amplitude voltage of an output pulse from the AND circuit 701 into a voltage for driving the gate of a driver transistor. A VDD power supply line 703 serves as the power supply of the logic circuit. A VHT power supply line 704 supplies the gate voltage of a driver transistor. A VH power supply line 705 serves as a power supply for driving a heater. A driver transistor 707 supplies a current to a heater 706. A GNDH line 708 receives a current flowing through the heater.
FIG. 11 is a block diagram for explaining a series of operations until the heater is driven after inputting a logic signal such as print data to the head substrate. In FIG. 11, the signal flow is schematically indicated by arrows. FIG. 11 shows a circuit block corresponding to one ink supply port.
In FIG. 11, the same reference numerals as those shown in FIG. 9 denote the same parts, and a description thereof will not be repeated.
An input circuit 104c includes a buffer circuit for inputting a logic signal to a logic circuit (to be described later) such as a shift register or decoder. A logic circuit 104b includes a shift register and latch circuit for temporarily storing externally input print data, and a decoder for outputting a block selection signal for selecting a plurality of heaters divided into desired numbers of blocks. The logic circuit 104b is arranged at the end of the head substrate. A logic circuit 104a includes at least an AND circuit which calculates the logical product between a block selection signal sent from a decoder and a print data signal output from a shift register via a latch circuit, and a voltage conversion circuit.
As is apparent from FIG. 11, the heater arrays 102 are arranged on the two sides of the ink supply port 101. The driver transistor array 103 is arranged along each heater array 102, and the logic circuit 104a is arranged along each driver transistor array 103.
When print data is input to the shift register via the input terminal 105, the shift register temporarily stores the print data, and the latch circuit outputs the print data signal. Then, a block selection signal for selecting a block of heaters divided into desired numbers of blocks, and a print data signal are ANDed. A current flows through a heater in synchronism with a heat enable signal HE which determines the current driving time. The series of operations is repeated for respective blocks to execute printing.
FIG. 12 is a plan view showing the layout of the head substrate shown in FIG. 9.
In FIG. 12, the same reference numerals as those shown in FIG. 9 denote the same parts.
As shown in FIG. 12, each driver transistor 103a of the driver transistor array 103 is a MOSFET corresponding to one heater 102a. 
A drain electrode D 103b of the MOSFET is series-connected to the heater 102a. The MOSFET has a gate electrode 103c and source electrode S 103d. In this layout, the heater 102a is adjacent to the driver transistor 103a, and the heater pitch and driver transistor pitch are equal to each other.
The term “pitch” is defined as a distance (interval) between a center of one constituent element and that of its adjacent constituent element in an arrayed direction of the constituent elements. In FIG. 12, an “X” axis indicates an arrayed direction of heaters and driver transistors. Thus, with respect to the arrayed direction “X”, the “pitch” of heaters indicates a distance (interval) A between the center of the heater 102a and that of its adjacent heater. As shown in FIG. 12, the center of the heater 102a means that the length indicated by an arrow a is equal to that indicated by another arrow a′. Likewise, with respect to the arrayed direction “X”, the distance (interval) between the center of the driver transistor 103a and that of its adjacent driver transistor is “B”. As shown in FIG. 12, the center of the driver transistor 103a means that the length indicated by an arrow b is equal to that indicated by another arrow b′. Note that the pitch A of the heater 102a is equal to the pitch B of the driver transistor 103a in the example of FIG. 12.
Recent inkjet printing apparatuses (to be referred to as printing apparatuses hereinafter) are increasing the arrangement density of printhead nozzles in order to achieve high-speed, high-quality printing. As a method of manufacturing nozzles at high precision has been developed, a nozzle pitch of about 600 dpi in actual size has been achieved.
In accordance with this nozzle pitch, heaters and driver transistors for driving them are formed on a silicon substrate. For example, for nozzles at a resolution of 600 dpi, heaters and driver transistors are arranged at approximately the same 600-dpi resolution. The driver transistor is often formed from a MOS transistor which controls a current flowing through the source-drain path by the gate application voltage. When arranging MOS transistors at the 600-dpi resolution, the gates of MOS transistors are arranged at high density regardless of the nozzle pitch, and then a plurality of MOS transistors are parallel-connected in accordance with the nozzle pitch, in order to implement an efficient arrangement. More specifically, as shown in FIG. 13, the printing apparatus adopts a circuit arrangement in which heaters at 600 dpi are driven using MOS transistors connected by juxtaposing, for example, four (4) gate electrodes.
FIG. 13 is a plan view showing part of the layout of a head substrate having a nozzle resolution of 600 dpi. In FIG. 13, the same reference numerals as those described above denote the same parts, and a description thereof will not be repeated. Reference numeral 103A denotes a driver transistor arrangement region; 107, a power supply line.
The driver transistors need to have a gate width W capable of supplying a current enough for ink discharge to the heater 102a. 
More specifically, it is necessary that the MOS transistor operates in a linear region upon supplying a heater current, and the ON resistance at this time is much lower than the heater resistance. For example, for conventional MOS transistors arranged at the 600-dpi pitch, four gates are arranged for one nozzle, so transistors having a relatively large gate width W can be formed. That is, a gate width W larger by four times than the width of the physical driver transistor arrangement region 103A can be implemented.
In the current printhead arrangement, the power supply line 107 for applying a power supply voltage to a heater is arranged on an upper layer above the transistor arrangement region. The power supply line needs to ensure a predetermined wiring width or more under the restriction on the parasitic resistance. In the conventional arrangement having a nozzle pitch of about 600 dpi, the wiring width is often larger than the originally required gate width W of one of juxtaposed transistors. Hence, the gate width of the driver transistor can be designed with a margin.
The method of manufacturing nozzles at high precision is being improved, and a higher density can be expected. For example, in an arrangement in which the nozzle density increases to about 1,200 dpi which is double 600 dpi, the number of juxtaposed gates of transistors per nozzle decreases from four to two, compared to the conventional arrangement with 600-dpi pitch.
FIG. 14 is a plan view showing part of the layout of a head substrate having a nozzle pitch of 1,200 dpi. Also in FIG. 14, the same reference numerals as those described above denote the same parts, and a description thereof will not be repeated. Reference numeral 103B denotes a driver transistor arrangement region.
When coping with high-density nozzles at a pitch of 1,200 dpi, the number of gate electrodes 103c which can be juxtaposed per nozzle is two. For this reason, only transistors having half the gate width W of the conventional 600-dpi (4-gate) arrangement can be formed. Only a region defined by power supply wiring cannot provide a necessary transistor gate width in the conventional 600-dpi arrangement, and transistors sometimes need to be arranged in a region exceeding the power supply wiring region. When arranging transistors in a region exceeding the power supply wiring region, it is required in terms of even cost to suppress the gate width W of the transistor as much as possible and downsize the head substrate.
As described above, as the nozzle density increases, it becomes difficult to design the gate width of the driver transistor integrated in the head substrate with a margin for characteristics. In a head substrate using driver transistors with little margin for the gate width, the design must pay attention to a change of the characteristics of the driver transistors. Particularly, in a printhead using a heater for a printing element, bubbling by heat generated by the heater is used to discharge an ink droplet, so heat of the heater greatly changes. The driver transistor is arranged near the heater, and the characteristics of the driver transistor are influenced by heat of the heater.
As examined above, driver transistors integrated in a conventional head substrate having a nozzle density of about 600 dpi (four gates) ensure a sufficiently large gate width, so the influence of heat is not particularly recognized as an important issue. However, to cope with higher nozzle density, a heat-tolerant head substrate needs to be designed. To reduce the influence of heat, the gate width W of driver transistors may be increased. However, this method increases the chip size on a head substrate having a nozzle density of 1,200 dpi, and raises the cost.
On the head substrate, a sensor for monitoring the temperature of the head substrate, an energy adjustment circuit for achieving stable driving, an electrode for an external electrical connection, and the like sometimes need to be arranged. In addition, recent printing apparatuses must increase the number of nozzles and prolong the nozzle array in order to increase the print speed. Therefore, the size of the head substrate increases in the nozzle array direction. If the head substrate size is large in the nozzle array direction, a relatively large temperature distribution tends to be generated in the nozzle array direction upon the print operation.
In a printhead using a head substrate which integrates heaters, it is particularly important to detect the substrate temperature because the ink discharge characteristic greatly changes depending on the substrate temperature.
In conventional temperature detection, a diode is arranged at the end of a head substrate or the like to obtain a temperature from the forward voltage. Alternatively, wiring is laid out in the nozzle array direction to obtain a temperature from a change of the resistance. When the diode is arranged at the end of the head substrate, the temperature of a local region where the diode is arranged is detected. When wiring is laid out in the nozzle array direction to obtain a temperature from a change of the resistance, the average temperature of the wiring is detected.
However, in a head substrate long in the nozzle array direction, the temperature distribution in the nozzle array direction is desirably measured to perform higher-precision driving control. If dedicated temperature sensors are arranged in the nozzle array direction to measure the temperature distribution in the nozzle array direction, such arrangement increases the head substrate size.
In the first place, the heater pitch and driver transistor pitch need not be aligned to each other, and the driver transistor can take a different pitch. However, if the heater pitch and driver transistor pitch are different from each other, the heater and driver transistor need to be connected by stepwise wiring or diagonal wiring. The wiring has layout rules such as the thickness and the interval between adjacent wires. For diagonal wiring, the interval between the heater and the driver must be widened, increasing the head substrate size. To prevent the increase in head substrate size caused by diagonal wiring, the heater pitch and driver transistor pitch must be aligned to some extent. For example, even if the driver transistor pitch can be set smaller than the heater pitch, the driver transistor pitch is set unnecessarily large. Neither a space which is generated if the driver transistor pitch is decreased can be ensured, nor can a functional circuit be arranged, resulting in an inefficient layout.
If the nozzle array becomes long, variations in heaters in the nozzle array direction, variations in discharge characteristic, or variations in the parasitic wire resistance from the power supply terminal to the heater become serious.
To correct such variations in the nozzle array direction, an energy adjustment circuit may also be arranged for each heater group in the nozzle array direction. When the energy adjustment circuit is arranged at the end of a head substrate, the scale of a circuit arranged at the end of the head substrate increases as the number of nozzles (the number of heaters) increases, resulting in a large head substrate size. The circuit arranged at the end of the head substrate and the heater need to be connected by wiring. As the number of heater groups increases, the number of wires increases. To ensure the wiring region, the head substrate size increases. To suppress the increases in the wiring region and the number of wires at the end of the head substrate, the energy adjustment circuit is desirably arranged in the nozzle array direction. Even in this case, the head substrate size increases in the conventional circuit arrangement.
As described above, when the temperature sensor, energy adjustment circuit, or the like is arranged in the nozzle array direction in a head substrate long in the nozzle array direction, the substrate size increases.